In recent years the digital transmission of information by means of high speed, high capacity pulse transmission systems has gained a foothold as a viable mode in long haul telephony applications. These systems are capable of carrying a few hundred megabits per second of information over repeatered coaxial cable lines. To maintain the integrity of the digital pulses over the total system from end to end, digital repeaters are placed at intervals dependent on the type of coaxial cable, the bandwidth of the transmitted information and the error rate specified for the total system. A digital repeater intercepts the incoming pulse streams to reshape and retime the dispersed pulses, thereafter retransmitting them onto the following cable interval.
An essential function in a repeater is performed by a threshold detector, which during an optimum time-window decides on the existence or absence of a pulse by sensing a voltage above a predetermined threshold. The accuracy of this decision largely determines the error rate of the system. Clearly such accuracy is a function of two variables, first the integrity of the incoming pulses that are ultimately presented to the threshold detector, and secondly on the position (in time) of the optimum time-window. This position is derived from the timing circuit which extracts a sinusoidal timing signal from the incoming pulse stream. The extracted sinusoid possesses the proper phase necessary for accurate timing, but varies in amplitude due mainly to the quasi-random occurrence of incoming pulses. A dense sequence of pulses produces larger amplitude than a not so dense sequence. There is, however, a lower limit set by the coding scheme of the multiplexing system, so that a repeater has to contend with only a variation in amplitude limited in most cases to between 10 and 20 dB.
In order to remove the amplitude variation from the timing signal, which is necessary for proper timing, the raw timing signal must be amplified and thereafter limited. As may be expected, the phase response of the limiter is of prime importance. It must be constant within preset tolerances over the region of amplitude fluctuation. Hence, limiting was accomplished by diodes in two consecutive stages superceded each by sufficient amplification and followed by yet a third amplifier stage. In an alternative, a series-shunt limiter was utilized which is preceded and followed by amplification. Such an arrangement is disclosed in the article by I. Dorros et al titled: "An Experimental 224 Mb/s Digital Repeatered Line", published in The Bell System Technical Journal, Vol. XLV, September 1966, Number 7, at page 1029. Under the heading "Limiting" in that article the authors teach that transistor limiting was avoided in order to keep amplitude-to-phase conversion at a minimum (i.e. the phase response constant). In that instance, as in other prior art limiters for the application in question, diode limiting is utilized.